Yet another Denford triac at fault!

[m_c]

And having looked at the datasheet, it could even be seals underneath the 4/2 valve leaking, as they mount onto a base which the hoses connect to.

I'd still start by swapping the pilot valves, and checking those seals. Then if the problem doesn't move, swap the valve bodies, and check the next layer of seals.

[jamesgates1000]

Swapped pilot valves, still same issue. Opened up the valve body and found a tiny square section o-ring that was creased, turned it upside down and now works. Can't seem to find a replacement seal set so will order a new valve body. Thanks for the help.

[m_c]

Sounds like a Quad ring, or x-ring, but by the time you find the right size, and somebody to supply them, a new valve body isn't that expensive. Plus the new valve should be guaranteed to work.